GENEVA (AP) — Prince Sadruddin Aga Khan, a wealthy philanthropist who held a string of top United Nations humanitarian posts and was the uncle of the spiritual leader of Ismaili Muslims, has died, associates said today. He was 70.

Sadruddin died Monday in Massachusetts General hospital in Boston, said Nasir Sunderji, an official of the Geneva-based Bellerive Foundation, an environmental organization founded by Sadruddin.

There were no immediate details on the cause of death.

Sadruddin started his long career with the UN as an adviser to UNESCO for Afro-Asian projects. He was both the youngest and longest-serving UN High Commissioner for Refugees, taking over the post in 1965 at the age of just 33 and remaining in office for 12 years.

"All in UNHCR and the humanitarian community are deeply saddened by the passing away of Sadruddin Aga Khan," said Ruud Lubbers, the current chief of the agency. "He left an indelible print on UNHCR history, leading the agency through some of the most challenging moments."

Sadruddin spearheaded UN efforts to cope with 10 million refugees from the breakup of Pakistan and creation of Bangladesh in 1971. He helped find homes for tens of thousands of Vietnamese who fled their communist homeland by in the mid 1970s, and for Asians kicked out of Uganda by former dictator Idi Amin.

Lubbers — who found out about the death while on a tour of West Africa — said Sadruddin's name was "synonymous with UNHCR."

After stepping down as refugee chief in 1977, Sadruddin held a series of other senior UN roles, including co-ordinator for the UN humanitarian assistance programs for Afghanistan from 1988-1990 and special UN representative for humanitarian assistance for Iraq and Kuwait after the Gulf War in 1990 and 1991. He was once mentioned as a possible UN secretary general.

One of his mottos was to keep a "cool head and warm heart without getting cold feet."

After he stood down from UN duties, he became increasingly involved in environmental protection of the Alps and rare Alpine birds. He was decorated with a long list of international awards, including the French Legion of Honour.

Sadruddin was born in Paris in January 1933 into a world of fabulous wealth as the son of Sultan Mohammed Shah, Aga Khan III — spiritual leader of the the Ismaili Muslims.

He held French, Iranian and Swiss passports and was educated at Harvard University — allowing him to proclaim himself a "citizen of the world."

But despite the luxury associated with his famous family, he also maintained that his lack of roots helped him to empathize with the plight of the displaced.

"If you cannot help the poor, you cannot save the rich," he once said.

Sadruddin was married for five years to a model, Nina Dyer, and their divorce in 1962 hit the headlines. But he quickly shed the playboy image and, on the whole, managed to avoid the gossip that dogged many members of the dynasty.

He shunned the racehorses, fast cars and diamonds favoured by his half-brother Aly, who was briefly married to the actress Rita Hayworth and who was the father of Karim — the Aga Khan IV.

Sadruddin once famously said that he disliked horse-racing, and he listed his hobbies as sailing, skiing, hiking and flying kites.

Urbane and eloquent, he and his Greek-born second wife, Catherine Sursock, were familiar but discrete figures on the Geneva social scene. He was passionate about Islamic art and archeology, as well as about bridging the understanding between different cultures.

"I was brought up in the Muslim religion," he told London's Daily Telegraph in a 1998 interview. "My father insisted that I learn the Qur'an and encouraged me to understand the basic traditions and beliefs of Islam but without imposing any particular views. He was an overwhelming personality but open-minded and liberal."

At his elegant Chateau de Bellerive on the shores of Lake Geneva, Sadruddin amassed a huge collection of priceless paintings, drawings and manuscripts from Turkey, Iran and India, dating from the 14th century.

"Fate uprooted my family from Iran over 130 years, ago," he said in the interview. "I liked the idea of trying to getting some things back and taking care of them."